Info

Lapse rates (K/km)

Clouds

Clouds

10 10"* 10" Cloud density (g/l)

Figure 4.24. Equilibrium cloud condensation model of Neptune's atmosphere (as Figure 4.7). Calculated cloud layers: H2O cloud (water, then ice) atp > 1,000 bar, NH4SH at ~50 bar, H2S ice at ~8 bar, and CH4 ice at ~2 bar. Assumed composition: O/H = 100x the solar value, N/H = the solar value, S/H = 11 x the solar value, C/H = 50 x the solar value.

solar value, N/H = the solar value, S/H = 11x the solar value, and C/H = 50 x the solar value. The low N/H value and the choice of other abundances will be discussed further in the following paragraphs. Considering the measured (or assumed) high abundances of these gases, the SALR can be seen to be very different from the DALR and thus the dry and wet temperature profiles differ significantly. The ortho:para hydrogen ratio was assumed to be in the "frozen" state outlined in Section 4.1.3.

The ortho:para hydrogen ratio in Neptune's atmosphere was determined from Voyager IRIS observations by Conrath et a/. (1998), who noted that Neptune has a fairly symmetric temperature structure in the upper troposphere, which is coolest at midlatitudes. These regions were found to correspond with a low fp, which is consistent with these being regions of rapid uplift, a deduction that is further confirmed by the observation that vigorous convective activity is seen at these latitudes. In the stratosphere, Fouchet et a/. (2003) found that fp decreases with height, but is significantly greater than the local equilibrium para-H2 fractions. Although the optical depth of Neptune's stratospheric hazes is known to be greater than those of Uranus (next section, "Clouds and hazes''), the particle sizes are estimated to be rather larger (Pryor et a/., 1992) and thus the number density of aerosols is less, leading to presumably less efficient catalysis. In addition, the eddy-mixing coefficient in Neptune's atmosphere is 10x greater than in Uranus' atmosphere leading to rapid vertical transport of high-/p air from the tropopause.

Observations of Neptune's composition profiles are listed in Table 4.9. Ground-based microwave observations suggest that the atmosphere of Neptune is, like Uranus, greatly depleted in ammonia by a factor of roughly 100 relative to solar down to levels of approximately 50 bar. This suggests again that large quantities of ammonia may be locked up in an aqueous ammonia cloud, or that the abundance of H2S exceeds that of NH3 by a factor of at least 5 and thus that the formation of an NH4SH cloud at —40 bar effectively removes all remaining ammonia from the atmosphere. The H2S abundance is estimated from these ground-based microwave studies to be (10-35) x the solar value. However, alternative explanations exist. For example, it may just be that Neptune (and Uranus) has been deficient in nitrogen since formation since N2 is not efficiently trapped in amorphous ice unless the ice formation temperature is very cold (see Chapter 2). However, it would appear unlikely that Jupiter and Saturn should be nitrogen-rich and Uranus and Neptune nitrogen-poor if these planets formed in the neighborhood of their current distances from the Sun. Instead, we would have to form Jupiter and Saturn initially at the edge of the solar system (at —30 AU) and then migrate inwards, and form Uranus and Neptune at 5 AU to 10 AU and then migrate outwards, which seems unlikely. Another explanation for the low abundance of ammonia in Neptune's atmosphere is that nitrogen may instead be mostly in the form of N2 in the observable atmosphere, which is difficult to detect spectroscopically. This scenario is consistent with the observed levels of stratospheric HCN (v.m.r. —1 x 10~9, Marten et a/., 2005), which is most likely formed from the photolysis by-products of CH4 and nitrogen atoms. High levels of molecular nitrogen, a disequilibrium species in the observable atmosphere of Neptune, suggest rapid convection, which is consistent with Neptune's strong internal heat flux, the non-equilibrium ortho:para hydrogen

Table 4.9. Composition of Neptune.

Gas

Mole fraction

Measurement technique

Reference

He

and assuming N2 v.m.r. <0.7%

Voyager far-IR Voyager far-IR ISO SWS/LWS

Conrath et al. (1991) Conrath et al. (1993) Burgdorf et al. (2003)

f a JeH2

0.63 </eH2 < 1.0

Visible hydrogen quadrupole

Baines et al. (1995b)

nh3

May be supersaturated (w.r.t. NH4SH) at p < 20-25 bar, hence greater than Uranus

Ground-based microwave Radio occultation

Ground-based microwave

de Pater and Massie (1985)

Lindal et al. (1992) de Pater et al. (1991)

h2s

(10-30) x solar, same for Uranus and Neptune

Ground-based microwave

de Pater et al. (1991)

S/N

>5x solar

Ground-based microwave

de Pater et al. (1991)

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